Applying Particle Models to Matter
As previously noted, we are pursuing two goals in Part 1 of this course. On the one hand, we want to get a solid understanding of energy and how we can use this understanding to get answers and make predictions about interesting phenomena. In Chapter 2 we got through the basics–introducing work and potential energy–and applied these concepts to mostly macroscopic phenomena. Now, in Chapter 3, we turn to the development of particle models of matter. We would like to be able to answer questions such as: Why do things melt and/or vaporize at different temperatures? What determines heat capacities of different substances? What aspects of these thermal properties are common to many substances and which are unique to particular substances? What common things can we say about all kinds of chemical bonding? Some of the most important ideas in our particle models of matter are related to the behavior of the spring-mass motion introduced in Chapter 2. We extend these ideas to understand the motion of atoms using a model that has at its core the idea that atoms and molecules in liquids and solids act like they oscillate exactly the way the spring-mass system oscillates. The relation between force and potential energy allows us to really make sense of the forces that act between atoms and molecules in terms of their equilibrium spacing and to understand the differences between solids, liquids, and gases in a much more fundamental way.
Looking back at Learning Expectations for you, the learner
Before going on, it might be helpful to think again about our expectations for you as a learner. After you have carried out the various activities in discussion/lab, had some intense discussions with your classmates, made several attempts at reading this chapter, and worked the Chapter 3 FNT (for next time) homework assignments, you should have a pretty good understanding of the ideas presented in this chapter. You won't understand many of the ideas presented in this chapter the first time you encounter them. This is normal–don’t give up. Your understanding will increase as you work with the ideas, talk to others about them, carry on a conversation with yourself about them, and apply them to new problems and questions. These Chapters in the course packet complement other course activities, and will be most useful if you refer to them at several stages during your work with this material. Things that make no sense at first will become clearer as you make more connections with other areas of knowledge, such as ideas from chemistry you are familiar with, and as you reorganize some of your thoughts in a little more consistent and logical way.
This is perhaps a good time to go back and reread several of the earlier parts of this course packet. You should definitely reread again, perhaps with a slightly different perspective now, the Preface on Meaningful Learning. Also, the Appendix M1: Science and the Place of Models in Science should begin to really make sense to you as we work our way through the three models in this chapter.
A famous Nobel laureate in physics, Richard P. Feynman, in the first chapter of an introductory physics book he wrote for Cal Tech students back in the late 50’s, claims that the particle model of matter is the most important or powerful model in science. Here is what he said:
“If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.” -The Feynman Lectures on Physics, Volume I, page 1-2
Your job over the next couple of weeks is to use “a little imagination” and apply a “little thinking” to the content of this powerful statement.
In This Chapter
The heart of the content in Chapter 3 is the development of a full understanding of the details contained in the Feynman quote. You already have a lot of useful ideas about this model. Much of it you have studied in chemistry. Keep consciously trying to integrate this new material with things you already know, like the ideal gas model (PV=nRT). It will take mental effort, but the understanding you gain will help your see chemical and biological concepts in a new light.
One of the areas in which our particle model of matter really shines is in explaining the experimentally observed thermal properties of matter, e.g., the values and trends of the specific heats of many substances in the gas, liquid, and solid phases. One of the interesting things about science is that it is in trying to resolve discrepancies that we push ahead and make breakthroughs. One of the discrepancies we will meet as we look at specific heats is that values for gases as well as solids are often lower than we would predict, especially at lower temperatures, but tend to rise to the predicted values as the temperature rises. The changes we need to incorporate in our model are due to the quantum mechanical nature of matter on a microscopic scale. We introduce some quantum ideas here and will continue to return to them throughout the course. Of course, you already know a lot about some of the central notions of quantum mechanics from your study of chemistry. For example, you have encountered the notion of orbital, or quantized energy levels, for the electrons swirling about the nuclei of atoms. When you get to the discussion in this chapter on how quantum mechanics alters things, you should definitely connect it to what you already know about quantization.
Through the activities in discussion/lab, those using a sophisticated computer simulation and those involving liquid nitrogen, springs and masses, etc., and through the activities you carry out at home, the particle model of matter described in Chapter 3 will become part of your mental tool kit that you regularly use as you answer scientific questions in the future.